1. Field of the Invention
The present invention relates to a method for producing reduced iron agglomerates by reduction of iron oxide agglomerates incorporated with carbonaceous material in a moving hearth reducing furnace.
2. Description of the Related Art
In a MIDREX process, which is known as a method for preparing reduced iron, a reducing gas produced by degeneration of natural gas is blown into a shaft furnace through a tuyere so that the iron ore or iron oxide pellets filled in the furnace are reduced in a reducing atmosphere. This method uses a large amount of natural gas, which is expensive, and requires degeneration of the natural gas. Thus, this method inevitably results in high production costs.
Recently, processes for producing reduced iron using inexpensive coal in place of the natural gas have attracted attention. For example, U.S. Pat. No. 3,443,931 discloses a process for producing reduced iron including pelletizing a mixture of powdered iron ore and a carbonaceous material, such as coal, and reducing iron oxide in a hot atmosphere. In this process, a given depth of iron oxide pellets incorporated with a dried carbonaceous material is fed into a rotary hearth furnace. The contents are moved and heated by radiant heat in the furnace to reduce iron oxide by the carbonaceous material. The reduced pellets are cooled by radiative cooling and are then discharged from the furnace by a discharging apparatus. This process has some advantages over the MIDREX process: use of coal as a reducing agent, direct use of powdered iron ore, and a high reducing rate.
Rolling, friction or dropping shock when the iron oxide pellets are fed into the reducing furnace, however, causes formation of powder from the pellets and the powder is fed into the furnace together with the pellets. The fed powder is deposited on the rotary hearth. Since the powder also includes the carbonaceous material, it is reduced together with the iron oxide pellets to form reduced iron powder. A fraction of the reduced iron is discharged with the reduced iron pellets from the furnace, but the residual fraction is squeezed into the rotary hearth surface by the discharging apparatus. The squeezed reduce iron powder is deposited on the rotary hearth surface without reoxidation. Reduced iron powder is further deposited during the rotation of the rotary hearth and gradually integrates with the previously reduced iron powder to form a layer of a large reduced iron plate.
According to the above U.S. patent, a mixture of iron ore, coal powder, and SiO.sub.2 is heated at 1,300 to 1,400.degree. C. on a base refractory to form a low-melting-point substance containing FeO and SiO.sub.2, and then the furnace is cooled to form a semi-melted hearth, in order to mechanically discharge the reduced iron plate by a discharging apparatus and to facilitate heat transfer from the hearth to the iron oxide pellets.
Such a construction of the hearth inevitably requires a long preparatory period prior to furnace operation. Since the temperature range in which the hearth material can be present in a semi-melted state is around 1,150.degree. C. and is narrow, the temperature of the hearth must be controlled to be uniform. When the temperature of the moving hearth is not uniform, the temperature is low at two ends of the moving hearth, and the hearth member is present in an unsticky solid state. Thus, the bulk hearth member separates when the reduced iron agglomerates are discharged by the discharging apparatus. When the surface of the moving hearth is cooled by radiative cooling from the discharging apparatus, the internal section of the hearth is hotter and more viscous than the cooled surface. Thus, the powder included in the agglomerates is squeezed into the internal section of the moving hearth from the surface. As a result, the powder forms a large reduced iron plate which cannot be easily discharged by the discharging apparatus. Furthermore, the powder is mixed with the hearth material composed of FeO and SiO.sub.2 to cause an increased melting point of the hearth material. Thus, the semi-melted state of the hearth and thus the smoothness of the hearth surface cannot be maintained.
A possible alternative method to this process is construction of a shaped or amorphous refractory on the base refractory. The overlying refractory, however, may be damaged by thermal shocks. Furthermore, the construction of the shaped or amorphous refractory is performed by human-wave tactic and requires a long working period.